I think what we have here is a confusion of what evolution really is, how a species changes over periods of time, how species derive from a common ancestor, and why. Hmmm….where did that ‘why come from?” Does it even belong in this argument? Can we sufficiently answer this question in the context of the basic question at hand, which pertains to the idea of an organism becoming more complex over time?
That in mind, ‘why’ must be left out, because the question itself connotes a biological answer, and not necessarily an ecumenical one.
Organisms, in an epic biological sense, do not really evolve. All organisms, even those within the same specie, are different. Each has an individual set of mutations in their genome, which give it its distinctive characteristics(relative size, veracity, health(to some extent), etc.) Some mutations, within the parameters of a given environment, are advantageous to the individual in that environment. Some are less advantageous.
The idea of evolution, in this sense, is that a given environment will favor the organism with a genome better adapted to that environment. This will promote the prime directive of any life form on earth, which is to generate offspring, and therefore perpetuate the advantageous genome.
Over a period of time, the genomic changes a specie undergoes will cause members of that specie acquire new characteristics. If the drift is sufficient, a new specie, which cannot directly breed with its parent specie, will come into existence. Exactly where this line of demarcation occurs, is unknown; but it marks a distinction between species. These ‘species’ are collections of individuals with predominately similar genomes, enough so that they will only interbreed with each other.
Scientists came to this conclusion through observation of the morphology of similar characteristics of distinct species which, though having distinctly different characteristics, have features very much in common. This morphological comparison can be traced through the fossil record, and develop what came to be known as an ‘evolutionary tree’, which, for all practical purposes seems to reduce to simpler and simpler ‘root forms’, or less complex organisms. This view held tightly for roughly 150 years.
With the advent of genetic research, however, this tree has become more dendritic, shuffling species around to new groups based on their genomic similarities, as well as their morphological traits. With this basis to pull from , we can make some assumptions:
1) Even though we have made an assumption that a process such as evolution is at work, we have no clear evidence of a completely new specie forming out of another under direct observation.
2) That said, there seems to be enough evidence in both morphological characteristics, genomic similarities, and the ‘trees’ derived from the classification using these tools, to give us a pretty good theory of not only the process itself, but the factors that could be at work behind the evolutionary process.
3) That organisms mutate has been observed, and that offspring are all mutations, and not perfect copies of parents, gives us insight into the a mechanism that can drive an evolutionary process.
So where does all of this leave us with the question? Evolution is a process, we can safely assume that the complexity we see around us is derived from this process. It does not tell us that this process is linear, for the driving process of evolution is only that an organism that is different may be better suited for a given environment than another. Therefore, it may not be relevant to the discussion of evolution in this manner. As well, species evolve, not individuals, in the manner in which we are using evolution. Individuals mutate, and the drift of mutation over time is how we define evolution.
History shows us that complexity vacillates between eras, and some eras show more diversity than others. Diversity, then, is a function of evolution more than complexity in this sense. So the better question would be, “do species/genus evolve more diversity over time. This is entirely dependent upon the world at any given state, and not necessarily on a linear approach of ‘simple to complex’. When it is advantageous in the world to have diversity, life will fill the niche(s) appropriately. When it is not, it will not.
Now, cumulatively, and mostly assuming the correctness of the fossil record, it does appear life has gradually become more complex over time, and that speciation and diversity are the endgame, so far.
Therefore, an answer can be put forth:
1) Over short periods of time, complexity/diversity in evolution is situationally dependent.
2) Over the grand scale of the 5 billion year history this is Earth, mutation/evolution appears to have generated more complexity and diversity within and between genus/species. The answer, then, is both yes and no.
This is all, however, speculation. It just happens to be supported by good evidence…..